1. Field of the Invention
The present invention relates to an offset printing paper with excellent color printability, particularly to an offset printing paper for newspaper.
2. Description of the Related Art
In recent years, printing technologies have made great progress including offset printing, color printing, high-speed mass printing, automation, etc. With the advance of technologies related to printing paper, improvement of its various physical properties is demanded from the workability and printability aspects.
Particularly, for offset printing, because a dampening solution is used with ink on a printing press, a stress applied to the paper is greater than conventional copperplate printing. For this reason, the offset paper requires to possess moderate water absorbency resistance (i.e., a size property).
There are two methods for giving a size property to printing paper, a method using an internal size and a method using an external size (surface size). The former is a method for including a chemical inside the paper by adding an internal size (e.g., a rosin size, an enhanced rosin size, a neutral rosin size, an alkylketene dimer (AKD), alkenyl succinic acid anhydride (ASA), etc.) in paper pulp slurry in wet-end. The latter is a method for coating the paper with a surface size (e.g., a styrene/maleic acid type copolymer, a styrene/acrylic acid type copolymer, olefin/maleic acid type copolymer, a urethane type copolymer, etc.) using a coating machine such as a size press or a gate roll coater after the paper is made.
The internal sizes, however, have many problems in terms of costs, quality and workability, etc. including (1) It is necessary to add a chemical in low-concentration pulp slurry, (2) A fixing amount of the chemical in pulp sheet varies, (3) Whitewater system is polluted, etc. The method using an external size does not have the above-mentioned problems, and thus is preferable.
For the printing paper, it is common that a water-soluble polymer such as a starch, a chemically engineered starch, polyvinyl alcohol (PVA) or polyacrylamide (PAM), etc. is surface-coated as a measure for improving the surface strength of the paper. Therefore, in the external size method a surface size should be used together with these materials.
For example, a method for using a surface size mainly comprising a water-soluble polymer containing a styrene type monomer (a styrene type surface size) together with a starch has been widely used. As examples, Japanese Patent Laid-open No. 1985-185894, Japanese Patent Laid-open No. 1993-247888, Japanese Patent Laid-open No. 1999-140791, etc. can be mentioned.
Surface sizes mainly comprising a water-soluble polymer containing an olefin type monomer (ethylene, propylene, isobutylene, diisobutylene, octene, decene, etc.) (olefin type surface sizes) are well known.
For example, a diisobutylene-maleic acid type surface size was disclosed in Japanese Patent Laid-open No. 1975-89603; diisobutylene-maleic anhydride type surface sizes were disclosed in Japanese Patent Laid-open No. 1980-67094 and Japanese Patent Laid-open No. 1985-9994; a surface size comprising a water-soluble polymer containing xcex1-olefin and (metha)acrylic ester is disclosed in Japanese Patent Laid-open No. 1990-200896, Japanese Patent Laid-open No. 1990-269895 and Japanese Patent Laid-open No. 1991-8894.
Because a surface size is basically a material having both a hydrohobic group and a hydrophilic group, it can cause foaming. Thus, a surface size which exhibits high size performance after using in a small amount is desirable.
An increase in color printing is also a remarkable phenomenon. As a result, the color printability of the paper (e.g., improvement of ink acceptability, etc.) has become one of the requirements as well.
Improvement of color printability by using a water-soluble polymer and a styrene type surface size together is disclosed in Japanese Patent Laid-open No. 1999-140791.
Newsprint (a roll of newsprint) printing paper mainly comprises mechanical pulp or deinked pulp (DIP). Although the newsprint is classified into medium/low-grade paper, it has to withstand more harsh conditions than regular printing paper because newspaper printing for the specified number of copies has to be done within a specified time period.
To improve paper strength of the newsprint, a starch, PVA or PAM is coated on. As a method for coating the newsprint, in Japan, normally a gate roll coater (GRC) which is a coat-forming transferring system capable of high-speed coating is used. Characteristics of the GRC system is briefly summarized for example in proceedings of the Japan Technical Association of the Pulp and Paper Industry Vol. 43, No. 4 (1989), p.36, the Paper Pulp Technical Times Vol. 36, No. 12 (1993), p.20 and others.
The object of the present invention is to provide offset paper, particularly newsprint for offset printing, which possesses a moderate size property (water absorbency resistance) and excellent color printability.
The inventors of the present invention found that mixing surface sizes such as a styrene type surface size and an olefin type surface size has synergistic effects on water absorbency resistance when coating on the a base printing paper (particularly, a base paper for newsprint) and drying the coating. The satisfactory size effect (an effect of providing water absorbency resistance) and excellent color printability can be achieved by coating a small amount. From these findings, the inventors have solved the objectives of the present invention.
The present invention relates to offset paper which is made by coating a surface-treating agent mainly comprising three constituents: Constituent A, Constituent B and Constituent C shown below, on a base paper for printing, and drying the coating.
Constituent A: A water-soluble polymer.
Constituent B: A surface sizing agent which is a copolymer of a styrene type monomer and an anionic monomer and which mainly comprises a soluble copolymer including 20 to 80 weight % of the styrene type monomer.
Constituent C: A surface sizing agent which is a copolymer of an olefin type monomer and an anionic monomer and which mainly comprises a soluble copolymer including 20 to 80 weight % of the olefin type monomer.